linux-kernel - Re: [PATCH v1 04/17] mm: let _folio_nr_pages overlay memcg

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Message-ID: <wi53ecg3o5eemp2hwy5sjbgoroulbmnbbbz6pub2ratbwrdhg3@pnhiy45qirr3>
Date: Wed, 23 Oct 2024 14:38:22 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	cgroups@...r.kernel.org, x86@...nel.org, linux-fsdevel@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Koutný <mkoutny@...e.com>, 
	Jonathan Corbet <corbet@....net>, Andy Lutomirski <luto@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v1 04/17] mm: let _folio_nr_pages overlay memcg_data in
 first tail page

On Thu, Aug 29, 2024 at 06:56:07PM +0200, David Hildenbrand wrote:
> Let's free up some more of the "unconditionally available on 64BIT"
> space in order-1 folios by letting _folio_nr_pages overlay memcg_data in
> the first tail page (second folio page). Consequently, we have the
> optimization now whenever we have CONFIG_MEMCG, independent of 64BIT.
> 
> We have to make sure that page->memcg on tail pages does not return
> "surprises". page_memcg_check() already properly refuses PageTail().
> Let's do that earlier in print_page_owner_memcg() to avoid printing
> wrong "Slab cache page" information. No other code should touch that
> field on tail pages of compound pages.
> 
> Reset the "_nr_pages" to 0 when splitting folios, or when freeing them
> back to the buddy (to avoid false page->memcg_data "bad page" reports).
> 
> Note that in __split_huge_page(), folio_nr_pages() would stop working
> already as soon as we start messing with the subpages.
> 
> Most kernel configs should have at least CONFIG_MEMCG enabled, even if
> disabled at runtime. 64byte "struct memmap" is what we usually have
> on 64BIT.
> 
> While at it, rename "_folio_nr_pages" to "_nr_pages".
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

BTW, have anybody evaluated how much (if anything) do we gain we a
separate _nr_pages field in struct folio comparing to calculating it
based on the order in _flags_1? Mask+shift should be pretty cheap.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov